1. Field of the Invention
The present invention relates to a gate pulse generator for controlling the triggering of a thyristor converter used in the direct-current transmission and more particularly to a gate pulse generator for a thyristor converter in which in order to fire thyristors of the thyristor converter in which each unit arm is composed of a plurality of series-connected thyristors, a gate pulse signal is generated in response to a gate command signal and a thyristor forward voltage signal and is converted into a light signal which in turn is transmitted to the thyristor converter.
2. Description of the Prior Art
In general, high-voltage thyristor converters used for direct-current transmission have a high voltage rating and a high current rating so that a unit arm comprises a plurality of unit thyristors connected in series or series-parallel. In this case, in order to attain the object of the series or series-parallel connections, gate triggering means must be so designed and constructed as to trigger a plurality of thyristors simultaneously. Furthermore, since a plurality of series-connected thyristors are at different potential levels, triggering means for respective series-connected thyristor units must be isolated from each other. As a result, recently light triggering methods utilizing optical transmission systems with a high degree of electrical isolation capability as well as a fast response have been widely used.
In the case of the light triggering method utilizing a light transmission system, on a high-voltage-side a pulse amplifier must be provided for converging a light pulse signal transmitted from the ground side to the high voltage side into an amplified electrical signal, thereby triggering all the thyristors in each arm of the thyristor converter. Furthermore a gate power supply circuit must be provided in order to supply the power for operating the pulse amplifier. The power for operating the gate power supply circuit is supplied through a snubber circuit (or a DC voltage grading resistor) provided for each thyristor. The capacity of the gate power supply circuit is dependent on the value (peak value) of a gate current to be supplied to each thyristor, a pulse duration and a number of pulses (normally one pulse per cycle and two or more pulses per cycle in the transient state). On the otherhand, the capability of supplying the power to the pulse amplifier is determined by the impedance of the snubber circuit and the capacitance of a capacitor connected to the power input side of the pulse amplifier in order to compensate for an instantaneous power supply failure. Moreover in order to ensure the stable supply of the gate current to each thyristor, the relation that the ability of supplying the input energy to the gate power supply circuit is higher than the output energy therefrom must be maintained.
It follows therefore that in order to make the gate power supply circuit compact in size, it is very important to decrease the required power capacity of the pulse amplifier to a minimum. However, it is necessary that the turn-on time interval of each thyristor must be shortened so as to reduce the variations in turn-on time of the thyristors so that it is impossible to reduce the value of the gate current to an extremely low level. Moreover, the number of pulses of the gate current is dependent upon the operation conditions of the system and is normally one pulse per cycle, but the capability of generating two or more pulses per cycle is required in the transient state.
Next, the prior art method for determining a time width of the gate current which affects the capacity of the high-voltage-side pulse amplifier will be described.
In order to investigate the gate pulse generation modes, the following three operation modes of the thyristor converter are considered:
(a) Mode A: a mode in which the thyristor converter operates normally as a rectifier or an inverter;
(b) Mode B: a mode in which the thyristor converter operates as a rectifier or an inverter and a direct current flows interruptedly; and
(c) Mode C: a mode in which when the thyristor converter operates as an inverter and a margin angle of commutation becomes insufficient, the forced trigger protection is effected.
In the case of the operation of the converter, the gate pulse generation modes can be fundamentally divided into the above-described modes A, B and C. Meanwhile, when the direct current is lowered almost to zero (but, unlike the case of the mode B, the direct current flows without interruption), the thyristor can be turned off. However, in this case, from the standpoint of supplying a retriggering pulse, the above-described phenomenon is substantially similar to the interruption of the direct current so that it may be regarded as the interruption of the direct current for the sake of convenience in analysis as will be described below.
Of the above-described three modes, in the mode B two gate pulses are generated per cycle while in the modes A and C only one gate pulse is generated per cycle. The normal operation corresponds to the mode A while the modes B and C temporarily result due to the external disturbance on the system. Especially the mode C results due to temporary phenomena such as the distortions of the system voltage and to an overload so that there exists almost no chance that the mode C results continuously. Therefore in the steady state, only the mode A is taken into consideration.
Next the pulse duration of the gate signal will be discussed. In the case of the mode A, the pulse width may be made very short because all the thyristors can be simultaneously fired, but in the cases of the modes B and C, the time points of the forward voltage generations vary from one thyristor to another due to the storage carrier difference .DELTA.Q between the thyristors so that the pulse duration must be increased by from several times to tens times as long as the pulse width in the mode A. In the mode A, the forward voltage signals of all the thyristors are already detected when the gate command signal appears so that the unbalanced voltages among the thyristors will not affect the triggering thereof and consequently all the thyristors can be triggered simultaneously. In the mode B, the gate command signal has been generated prior to the detection of the forward voltage signals of the thyristors so that the gate pulse is generated at a time point (the zero voltage point) at which the thyristor voltage most quickly changes from the reverse voltage to the forward voltage; that is, at a time point when the forward voltage is detected first. In this case, there exists a voltage difference due to the storage carrier difference .DELTA.Q among the thyristors so that a time difference .DELTA.t.sub.2 exists among the zero points of the thyristor voltages. In this connection, in the case of the mode A, a time difference .DELTA.t.sub.1 among the zero voltage points is almost zero. In the mode B, because of the time difference .DELTA.t.sub.2, when the gate pulse is generated by the forward voltage signal of the thyristor which rises quickly, the pulse duration must be selected longer than .DELTA.t.sub.2. As described above, in the mode C, the forced triggering is effected when the margin angle is insufficient in the case of the operation as an inverter. In this case, the phenomenon in the mode C is substantially similar to that in the mode B and when the forward voltage appears first during the time interval when the gate command signal continues due to the insufficient margin angle, the gate pulse is generated. Therefore, even in this case, the thyristor voltages are unbalanced due to the storage carrier difference .DELTA.Q among the thyristors so that a time difference .DELTA.t.sub.3 exists between the earliest and the latest time points at which the voltage passes the zero point. The value of the direct current Id is greater and also, in the case of commutation, the decrease rate of the current dI/dt is greater in the mode C than in the mode B. The storage carrier difference .DELTA.Q is greatly dependent upon the direct current Id and its decrease rate dId/dt. That is, the higher the direct current Id and its decrease rate dId/dt, the greater the difference .DELTA.Q becomes. As a result, .DELTA.t.sub.2 &lt;.DELTA.t.sub.3 and in the above described three cases, there exists a relation that .DELTA.t.sub.1 &lt;.DELTA.t.sub.2 &lt;.DELTA.t.sub.3. Although these differences vary widely depending upon the rating of the converter, the rating of the thyristors, the operating conditions and so on, in the case of the high-voltage converter for the direct-current transmission, the three time differences are as follows:
.DELTA.t.sub.1 .perspectiveto.0 PA1 .DELTA.t.sub.2 .perspectiveto.tens microseconds PA1 .DELTA.t.sub.3 .perspectiveto.from tens microseconds to hundreds microseconds.
So far the above-described facts are taken into consideration when the pulse duration T.sub.p of the gate pulse (refer to the signals a and b) is determined in accordance with the third case i.e. mode C. In practice, T.sub.p =200-300 microseconds.
Meanwhile, the peak value of the current i.sub.L of a light-emitting diode connected to the output of a gate pulse generator is selected hundreds mA-1 A in the indirect light triggering system in order to reduce the variations of the gate currents applied to the thyristors. On the other hand, the direct light triggering system in which a converter is composed of light-activated thyristors so that the triggering control is effected directly in response to a light gate signal, the peak value is of the order of a few amperes.
As is well known in the art, the life of light emitting elements such as light-emitting diodes is relatively short so that the conditions for attaining the driving current having a peak value of from hundreds mA to 1 A and a pulse duration of from 200 to 300 microseconds as described above are extremely severe. In this connection, the conditions for firing the thyristors by the above-described prior art gate pulse generator are considerably limited from the standpoint of a life of light-emitting elements and a capacity of a triggering power supply on the high voltage side and therefore present difficult problems in the design and fabrication of the thyristor converters and systems.